The present invention relates to an image recording apparatus wherein an image is recorded on a recording sheet wound around a drum to cover its circumferential surface.
There has been known an image recording apparatus wherein an image is recorded on a recording sheet such as a recording paper which is wound, with its recording surface facing outward, around a drum to cover its circumferential surface.
For example, there are given those wherein the recording sheet is a silver halide photographic light-sensitive material and laser exposure is used for recording an image and those wherein the recording sheet is a thermosensitive paper and laser exposure is used for recording an image.
In these image recording apparatuses, a method to exfoliate the recording sheet wound around a drum after image recording has been one wherein grooves are provided on the circumferential surface of the drum to be in parallel with the direction of drum rotation and exfoliation claws (peeling claws) are brought into contact with the grooves to exfoliate or peel the recording sheet from the drum.
As a method to fix on a drum a recording sheet wound around the drum, there has been known a method wherein a recording sheet is fixed on the circumferential surface of the drum by means of vacuum suction which is performed through a plurality of through-holes provided on the circumferential surface of the drum. On the circumferential surface of the drum, there are provided suction-communicating grooves through which the through holes are communicated or connected with each other so that the entire surface of the recording sheet on the circumferential surface of the drum may be attracted uniformly.
When a recording sheet is supplied to a drum to be wound around the drum, the leading edge of the recording sheet needs to be positioned on the circumferential surface of the drum. As a method of the positioning, there have been known a method to position by means of a roller which is pressed on the drum surface, and a method wherein a positioning member which nips a recording sheet is provided at the prescribed position on the drum for positioning.
As the latest trend in an image recording apparatus, the speed of revolution of a drum is accelerated for the purpose of shortening the time required for image recording. In addition, for the purpose of obtaining high image quality of the recorded image, a recording sheet such as a recording paper needs to be fixed on the drum surface smoothly.
However, when a conventional drum equipped with grooves which are to be touched by exfoliation claws and are in parallel with the rotary direction of the drum is rotated at high speed, the grooves which are to be touched by exfoliation claws serve as a path for the high speed air flow, resulting in occurrence of swelled portions each having a width identical mostly to that of the groove. Therefore, unevenness is caused on the recording sheet, and image quality is lowered accordingly. Further, when the swelling of the recording sheet is increased, negative pressure for suction through the through-holes on the drum is deteriorated, resulting in apprehensions that the recording sheet would come off the drum.
There is further a problem that when supplying a recording sheet to the drum, a leading edge of the recording sheet enters a conventional suction-communicating groove provided on the drum and the recording sheet fails to be supplied.
With regard to positioning of the leading edge of a recording sheet on the circumferential surface of a drum, a leading edge regulating member such as a roller or a positioning member is brought into contact with the drum in the conventional method. Therefore, the structure is complicated and further, adjustment is necessary in assembling.
The invention has been attained in view of the problems mentioned above.
The first object of the invention is to prevent occurrence of unevenness on the recording sheet surface and accidental swelling of the recording sheet, and to realize stable fixing of the recording sheet on the drum, and thereby to obtain outputted images having high image quality.
Further, the object is to make it possible to supply a recording sheet to a drum by preventing erroneous supply of the recording sheet to the drum, to make it possible to realize surely, with a simple structure, the positioning of the leading edge of the recording sheet on the circumferential surface of the drum, and to make the fixing of the recording sheet on the drum to be certain.
There has been known an image recording apparatus wherein a drum is rotated at a low-speed to make the recording sheet on which images are recorded to be wound around the drum to cover, on a close contact basis, the outer circumferential surface of the drum by a squeeze roller, then the drum is rotated at a high-speed to write images, and the drum is rotated at a low-speed to supply and eject a recording sheet.
In this image recording apparatus, a mechanism for low-speed rotation needs to be separated when high-speed rotation is made, resulting in the complicated mechanism, because a drum has been switched for high-speed rotation and low-speed rotation.
The second object of the invention is to provide an image forming apparatus wherein a drum can be rotated at a low-speed by a simple mechanism when supplying and ejecting a recording sheet, and to provide an image forming apparatus wherein a squeeze roller can surely be driven from a drum, or a drum can surely be driven from a squeeze roller.
There has been known an image recording apparatus wherein when supplying a recording sheet to a drum, the leading edge of the recording sheet is positioned by a positioning member provided on the drum side, and the recording sheet is exfoliated after completion of writing of images on the recording sheet.
In the image recording apparatus mentioned above, however, in the case of a positioning member located on the part of a drum, when the drum is rotated at a high-speed, dynamic imbalance is easily caused to cause a vibration, and the positioning member is hard to be machined, and it is difficult to position accurately the center of gravity of the drum on the rotary shaft of the drum.
The object of the invention is to provide an image forming apparatus wherein rotation of a drum at a high-speed is stabilized, and thereby the leading edge of a recording sheet can be positioned accurately.
As an image recording apparatus to record images on a recording sheet, there has been known one wherein a recording sheet is wound around a rotary drum and images are formed on the recording sheet wound around the rotary drum. In particular, there has been known one wherein a rotary drum which is hollow and is provided with plural holes each being passing through from the circumferential surface to the inside and rotates while holding the wound recording sheet, and a decompressing means which decompresses the inside of the rotary drum are provided, and the recording sheet wound around the rotary drum to cover its circumferential surface is brought into close contact with the rotary drum by decompression of the decompressing means so that images are recorded on the recording sheet.
In an image recording apparatus of this type, depending on a size of a recording sheet, suction force by means of the decompressing means applied on the recording sheet is varied as an edge position of the recording sheet is deviated from the regular position, or in accordance with the pressure inside the drum, and depending upon the speed of revolution of the drum, centrifugal force applied on the recording sheet and exfoliating force caused by an air current exceed the suction force to exfoliate the recording sheet from the rotary drum, which is a problem.
The fourth object of the invention is one to solve the problems mentioned above, which is to increase sharply the number of sizes of the recording sheet which can be selected by keeping the well balance between the suction force by means of the decompressing means which varies depending on the size of a recording sheet and is applied on the recording sheet and a group of centrifugal force which varies depending on the speed of revolution of the rotary drum and is applied on the recording sheet and exfoliating force caused by an air current and without lowering quality of recorded images.